For many years, reports have been published describing the discovery and investigation of an additional vascular system in mammals. This primo vascular system (PVS) is distinct from the blood and lymph vascular system and consists of primo nodes (PNs) and primo vessels (PVs) as its main constituents. We investigated samples of the PVS from the intestine surface of rats and observed several instances of a red thread-like structure (RTLS) and a red oval or round structure (RORS) in PVs and PNs, respectively. We conclude that the RTLS and RORS are most likely due to erythrocytes, indicating the occurrence of extramedullary hematopoiesis inside the PVS of the intestine surface. To the best of our knowledge, this is the first report showing detailed microscopic images of an RTLS traversing four PNs and a single PV. Our report is intended to document our findings and also to motivate others to repeat and extend our study in order to investigate in detail the possible extramedullary hematopoiesis occurring inside the PVS
